The mean ionic activity coefficient of NaBr in the (water + DL-valine + NaBr + K3PO4) system was determined by potentiometry at T = (298.2 and 303.2) K by using two ion-selective electrodes, (Na+) against (Br ). The maximum concentrations of sodium bromide, potassium phosphate, and DL-valine were (1.0, 0.01, and 0.4) mol.kg(-1), respectively. The activity coefficient of DL-valine was evaluated from mean ionic activity coefficients of NaBr by using a cross-differential thermodynamic relation. (C) 2013 Elsevier Ltd. All rights reserved.